1. Field of the Invention
The present invention relates to a projection optical system, exposure apparatus, and device fabrication method.
2. Description of the Related Art
As an optical material (glass material) of a projection optical system or an illumination optical system of an exposure apparatus, synthetic silica glass (synthetic quartz glass) having a high transmittance even in a vacuum ultraviolet region is used. The optical performance of this synthetic silica glass includes not only a transmittance to the wavelength of the exposure light, but also the refractive index uniformity, birefringence, and durability. In particular, it is important not to degrade the refractive index uniformity in order to ensure the exposure performance of the exposure apparatus for a long time period.
On the other hand, when irradiated with ultraviolet light for a long time, the synthetic silica glass changes the refractive index owing to compaction or rarefaction. “Compaction” is a phenomenon in which the density of optical glass in an irradiated region increases by ultraviolet irradiation, and the refractive index of the optical glass (irradiated region) rises in accordance with this density change. “Rarefaction” is a phenomenon in which the density of optical glass in an irradiated region decreases by ultraviolet irradiation, and the refractive index of the optical glass (irradiated region) lowers in accordance with this density change.
Techniques that cope with this refractive index change caused by compaction or rarefaction have conventionally been proposed. Japanese Patent Laid-Open No. 2008-63181 has disclosed a technique that suppresses compaction and rarefaction by controlling the concentrations of hydrogen molecules and OH groups to be contained in synthetic silica glass when manufacturing it. Japanese Patent Laid-Open No. 2005-114881 has disclosed a technique that reduces the influence of compaction on the imaging performance by forming a projection optical system by combining a lens using synthetic silica glass as a glass material and a lens using fluorite as a glass material. Japanese Patent Laid-Open No. 2006-73687 has disclosed a technique that reduces the influence of compaction on the imaging performance by forming a projection optical system by using synthetic silica glass preirradiated with an optical energy higher than an optical energy to be actually radiated per unit area. “Verification of compaction and rarefaction models for fused silica with 40 billion pulses of 193-nm excimer laser exposure and their effects on projection lens imaging performance, Proc. SPIE Optical Microlithography XVII Vol. 5377, 2004 (to be referred to as “reference 1” hereinafter)” has disclosed a technique that forms an optical system by combining a compaction-dominant material and rarefaction-dominant material.
In the technique disclosed in Japanese Patent Laid-Open No. 2008-63181, however, synthetic silica glass that reduces the occurrence of compaction can be manufactured, but it is difficult to assure the exposure performance of an exposure apparatus for a long term owing to the influence of residual compaction.
Also, the technique disclosed in Japanese Patent Laid-Open No. 2005-114881 can reduce the influence of compaction on the imaging performance by appropriately installing the lens using fluorite that does not cause compaction as a glass material. However, fluorite contains intrinsic birefringence that causes hardly correctable aberration in accordance with the polarized state of exposure light. This increases the design difficulty when designing a projection optical system. In addition, the manufacturing cost of the projection optical system rises because fluorite is expensive.
Furthermore, in the technique disclosed in Japanese Patent Laid-Open No. 2006-73687, an optical energy is to be radiated for a long time before processing the shape of an optical element (lens) of a projection optical system. This prolongs the manufacturing time of the projection optical system, and as a consequence raises the manufacturing cost.
Also, reference 1 has no practical description on the combination and layout of the compaction-dominant material and rarefaction-dominant material.
As described above, it is practically difficult for the conventional techniques to provide a projection optical system that reduces deterioration of the imaging performance caused by compaction or rarefaction, and secures the exposure performance of an exposure apparatus for a long time period, while decreasing the manufacturing cost.